Misplaced dedication and fear of letting other doctors down are among reasons the researchers cited as possible explanations.
Dr. Anupam Jena, a medical resident at Massachusetts General Hospital in Boston, developed food poisoning symptoms halfway through an overnight shift last year, but said he didn't think he was contagious or that his illness hampered his ability to take care of patients.
Jena, a study co-author, said getting someone else to take over his shift on short notice "was not worth the cost of working while a bit sick." He was not among the survey participants.
The researchers analyzed an anonymous survey of 537 medical residents at 12 hospitals around the country conducted last year by the Accreditation Council for Graduate Medical Education. The response rate was high; the hospitals were not identified.
The results appear in Wednesday's Journal of the American Medical Association.
Nearly 58 percent of the respondents said they'd worked at least once while sick and 31 percent said they'd worked more than once while sick in the previous year.
About half said they hadn't had time to see a doctor about their illness.
Dr. Thomas Nasca, the accreditation council's CEO, said residents are trained to put patients' needs above their own but also should recognize that if they're sick, their patients' would be better served by having another doctor take care of them.
Residents' hands-on postgraduate training is rigorous and demanding. Many work up to 80 hours a week and sometimes 24 hours a day in hospitals. The atmosphere in some programs is ultra-competitive, and residents may work while sick because they don't want to be seen as slackers, Jena said.
The council, which accredits hospital residency programs, has proposed revisions on residents' work hours and time off to reduce sleep deprivation and the chances for medical errors. The work week limit would remain at 80 hours.
The revisions, to be voted on by the council's board later this month, also call for residents and faculty not to ignore signs of illness and fatigue in themselves and colleagues and make sure that they're fit for duty. That might help reduce the reporting-while-sick problem, said study co-author Dr. Vineet Arora, associate director of the University of Chicago's internal medicine residency program.
A growing push to require flu shots for health workers also could help reduce the number of junior doctors who work while sick.
The federal Centers for Disease Control and Prevention strongly recommends annual flu vaccinations for all health care workers to prevent their patients from getting sick. And last week, the American Academy of Pediatrics endorsed mandatory flu vaccinations for all health workers.
Jena said flu "is really the main worry," because it's so prevalent and so easily transmitted.
Dr. Lauren Hughes, a former president of the American Medical Student
Association and a first-year resident in Seattle, said her program directors have warned residents "to take extra special care of ourselves" during the approaching flu season and winter months, and to stay home when sick.
"You can't take care of other people if you don't take care of yourself," Hughes said.